The present invention relates to a paste type thin electrode for a battery, in which the cost is reduced and the high rate discharge characteristics and the cycle life are improved, and to a secondary battery using this electrode.
At present electrodes for batteries, used commercially for secondary batteries, are broadly categorized as sintered type electrodes and non-sintered type electrodes. In the sintered type electrodes, active material is filled into a highly porous three dimensional substrate where metal powder is sintered to have a large porosity on both sides of a two dimensional metal substrate. In the non-sintered type electrodes, the active material powder with a binder is coated on a two dimensional metal substrate or grid, or filled into a three dimensional substrate, such as foamed nickel, metal bag or tube, without employing a sintered substrate.
In general, the former exhibits excellent characteristics in electronic conductivity (high-rate charge and discharge characteristics) due to a large amount of metal used in the sintered plaque and has a long cycle-life with excellent mechanical strength and stability in the shedding of active material, while it has the defect that the electrode is heavy and has a small volumetric energy density due to a small amount of active material impregnated therein because of a large volume of the electrode substrate.
On the contrary, a representative and simple non-sintered type electrode is inexpensive and light weight, and has a large volumetric energy density because of using an inexpensive substrate of a small volumetric amount in the electrode, which is easy to manufacture, through the coating or direct filling process of active material powder, while it entails the problem that the entire electrode is inferior in current collection ability as a whole, in the mechanical strength and in the holding of the active material. These are significant problems in secondary batteries where charging and discharging is repeated and, therefore, a variety of ideas are incorporated into respective battery systems.
As a result, non-sintered types have a variety of substrates to improve the above problems, as represented by a paste type or an application type, wherein active material powder is mixed with conductive material or a binder which is then mixed together with a solution and the obtained paste or slurry is coated on a two dimensional substrate of a variety of shapes, or in some cases the active material powders are filled in a pocket type or a tube type substrate which has innumerable small pores for electrochemical reactions.
As examples of non-sintered type electrodes, which are of the former type, a cadmium negative electrode, a metal hydride negative electrode for alkaline storage batteries, the positive and negative electrodes for lithium ion batteries and the positive and negative electrodes for lead acid batteries are cited. Non-sintered type batteries which are of the latter type are, for example, employed in part of the nickel positive electrode for large scale alkaline storage batteries or for certain types of lead acid batteries. As a substrate of the electrodes described herein, punching metal, a metal screen, foamed metal, a metal grid or the like are individually utilized according to the battery systems or the purpose.
However, recently, new types of electrodes in which a paste of active materials is filled into a foamed nickel porous substrate or into a nickel fiber substrate, which have a three dimensional structure, in the high density (hereinafter abbreviated as 3DM type), have started being employed as proposed in U.S. Pat. No. 4,251,603, which belongs to another non-sintered type in classification. However, though these types of electrodes have a high capacity and a high reliability and are easily made to have higher capacity and to be lighter weight compared with the sintered type, due to a small amount of metal employed in the substrate, they have the technical problems that the mechanical strength is low and the electronic conduction of the entire electrode is inferior due to a large pore diameter within the substrate and, in addition, have the technical problem that the cost of the substrate is high.
Since the present invention of paste type electrode relates to an improvement of the three dimensional substrate used in the above described 3DM system, in particular for alkaline storage battery system currently, for the convenience of the detail technological description of prior art, a nickel positive electrode for a small sealed cylindrical Ni/MH batteries is focused on thereafter.
As for the nickel positive electrode for alkaline storage batteries, the sintered type electrode, which was developed in Germany during the Second World War, has a high performance and is durable, which replaced the previous non-sintered type electrode, that is to say, the pocket type electrode, and, therefore, a sintered type electrode started to be used for rectangular Ni/Cd batteries requiring high performance and high reliability. As for the negative electrode, a similar conversion to the sintered type has occurred. As for the electrodes of sealed cylindrical Ni/Cd batteries developed afterwards, sintered type positive and negative electrodes have become the most popular because they are easily processed into thin electrodes. The small sealed cylindrical batteries represented by this nickel-cadmium battery (Ni/Cd battery) have achieved a dramatic growth as a power supply for portable compact electronic equipments, such as camcorders or CD players, which have achieved a remarkable growth in Japan starting in the 1980""s. However, in the 1990""s, a new type of nickel-metal hydride storage battery (Ni/MH battery) and a lithium ion battery successively have been put into practical use so as to begin expansion into the market of nickel-cadmium batteries.
And, as for a new market, applications for power supplies as power tools, applications for mobile power supplies, that is to say, for electrical vehicles (EVs), hybrid electrical vehicles (HEVs), electric power assisted bicycles or the like have newly started growing in recent years, and for those power supplies mainly Ni/MH batteries have started being used. A nickel positive electrode is employed for the positive electrodes of the above described Ni/Cd batteries and Ni/MH batteries for which the growth recently has been remarkable and the sintered types and 3DM types are used respectively, according to the applications under the present circumstances.
As for the structure of this nickel positive electrode for a mass-production level, the non-sintered type was limited only to the pocket type, due to the electrode mechanical stability. The pocket type electrode has a structure wherein active material powder is filled into a electrolyte proof metal bag with innumerable small pores to prevent the shedding of active metal powders as described above. The sintered type adopts a structure wherein a solution of active material salt is impregnated into the space of a three dimensional sintered plaque, followed by the process of conversion to the active material with alkaline solution. Naturally, the active material in this case is not in a powder condition.
Another non-sintered 3DM type, which is different from the pocket type, is reported as a nickel positive electrode employing foamed nickel in the ECS Fall Meeting (Detroit) Abstract No. 10 in 1981. This electrode has a structure using a foamed nickel porous body as a substrate, into which active material powder is filled.
Though a light weight nickel positive electrode with a high capacity is realized by using this foamed nickel as a substrate, it has the problems that the high power drain of the entire active material is not sufficient due to the large diameter of the internal spherical space, which is approximately 450 xcexcm in the case of even the smallest diameter, and it is expensive. Therefore, batteries using a sintered type nickel positive electrode which exhibit excellent characteristics in high-rate discharge are still the most popular for applications requiring high power drain.
However, the following shortcomings of the sintered type electrodes for those applications have been increasing, as problems in practical use, while applications are expanding, and, therefore, the introduction of the paste type electrodes are desirable. The shortcomings are: small energy density; heavy weight; large self-discharge due to the well-known shuttle reaction between nitride and nitrate ions, which is not present in the non sintered type. Since those applications require a high-rate discharge, thin electrodes are, in general, employed to increase the electrode surface area in order to have a large active area, which also increases the area of the substrates of the electrodes. Accordingly, a two dimensional substrate or a three dimensional substrate of low cost are particularly required and also light weight is a prerequisite for these high-power uses.
Therefore, new structures of three dimensional substrates to replace expensive foamed nickel such as in the 3DM type, which is a kind of the paste type of light weight, are proposed as follows:
(1) One sheet of electrode is formed by overlapping a plurality of extremely thin electrodes wherein active material powder is coated on the porous substrate, such as thin punched metal and foamed metal.
(2) Innumerable pieces of metal in the form of bristle or whisker are attached to a porous substrate, such as metal foil and punched metal (U.S. Pat. No. 5,840,444).
(3) Burrs are provided on a metal plate in the direction of the thickness of the plate (U.S. Pat. No. 5,543,250).
(4) A metal plate is processed to have a three dimensional corrugated form. Holes with burrs are provided on the crests of the corrugated form so as to increase the three dimensional shape (U.S. Pat. No. 5,824,435).
The structures or the substrates in the above described (1) to (4), however, have not solved all of the problems. In (1), there still remains the problem of the active mass shedding of each thin electrode due to the swelling of the active material during charge and discharge cycles, which essentially cannot be prevented. In (2), the thickness of the paste layer lacks uniformity due to the low binding strength between the metal fiber in bristle or whisker, or due to the non-uniformity of the holes of the substrate itself with respect to its characteristics and, additionally, it costs more than the conventional substrates. In (3), the structure is basically not three dimensional and, therefore, it has problems in the shedding of the active material powders following a decay in charge and discharge characteristics. In (4), though the above described problems have been improved to some extent and low cost can be expected, there still remains the problem that a desired three dimensional substrate shape is difficult to maintain. Because, the substrate of the corrugated form is easily expanded in the direction of the wave form during the electrode press work, which leads to the problem that the active material is easily peeled off from the substrate when it is wound into an electrode of a spiral form or when charging and discharging are repeated.
In addition, power supplies for electric power tools are desired, derived from how power tools are used, to have high-rate discharge characteristics, and the batteries for power-use, such as electric vehicles (EVs), hybrid electric vehicles (HEVs) and electric power assisted bicycles are desired to have improved high-rate discharge characteristics, particularly desired to be smaller and to be lighter in order to secure space within the vehicles and in order to improve fuel efficiency respectively, that is to say, to increase volume energy density (Wh/l) and gravimetric energy density (Wh/kg).
The present inventor solved the above described problems by forming an electrode for alkaline storage batteries as an application example as follows:
(a) Forming a conductive electrode substrate from a metal foil which is provided with innumerable concave and convex hollow parts or forming the same shape metal substrate by the metal deposition through an electrolytic method;
(b) Adjusting the thickness of the above described electrode substrate to substantially the same thickness as that of the electrode;
(c) For limiting the above described substrate to become two dimensional, partially or as a whole, by the electrode press work after filling the paste of active material powders as the main material, arranging the position of said concave and convex parts of the conductive electrode substrate to maintain the current collection ability of the whole electrode; and
(d) Preventing the peeling of the active material powders layer from the substrate through the spirally winding process of the electrode and also the shedding of active material powders that formed the electrode through the repetitive charging and discharging afterwards, by bending the walls of the concave and convex hollow parts into one direction specifically in the vicinity of the edge, just as to wrap the space between the concave and the next concave or the convex and the next convex in order to prevent the shedding of the active material powders.
In addition, by maintaining all the active material powder, within 150 xcexcm in the distance from the nearest conductive electrode substrate, the charging and discharging reaction, particularly the high rate discharge reaction, of the active material powder is enhanced and by using a cylindrical battery case wherein a ratio (t2/t1) of the thickness (t2) of the bottom to the thickness (t1) of the side walls is 1.5 or more, that is to say, by using a case of which the side walls have become thinner, the secondary battery is further made lighter and made larger in capacity.
Though the present invention is not particularly limited to a nickel positive electrode, in the case of application for a nickel positive electrode, in particular, a thinner nickel positive electrode is provided in which the thickness is 500 xcexcm or less for alkaline storage batteries, and the electrode uses an inexpensive, light weight and conductive metal substrate that can be formed only through mechanical operations on a metal foil or only through electrolytic metal deposition on the same pattern, without sintering or plating, resulting in excellent characteristics in charge and discharge characteristics, restraining the shedding of active material powder and light weight. Therefore, an inexpensive, light weight sealed cylindrical or prismatic nickel-metal hydride battery (Ni/MH battery) that shows excellent characteristics of high-rate charge/discharge and long cycle-life is achieved.